The object of the present invention is the preparation of acetic acid and/or methyl acetate, by using an isomerisation reaction of methyl formate.
Various means of access to acetic acid are known and used industrially. Amongst these are the carbonylation reactions of methanol. This carbonylation reaction may notably be carried out in the liquid phase, under the pressure of carbon monoxide, which is one of the reagents, in the presence of a homogeneous catalytic system comprising a compound based on rhodium and/or iridium and an iodinated promoter.
Another means of access is constituted by the isomerisation reaction of methyl formate in the presence of a catalyst based on rhodium or iridium.
In the isomerisation methods catalysed by iridium, the reaction is carried out with a solvent which is selected from the carboxylic acids, and more particularly, the acid produced. The reaction is moreover carried out under an atmosphere comprising nitrogen. It has in fact been noted that the carbon monoxide did not bring about any particular advantage during the reaction and could even be the cause of a certain inhibition of the isomerisation reaction, it favouring side reactions. It is to be noted that such a behaviour is entirely different from that which is observed when the catalytic system is rhodium-based, in which case the presence of carbon monoxide is essential for keeping the metal in the homogeneous phase. This type of method, whose interest is not questioned, does not have any real interest industrially since the reaction which is described therein is not sufficiently efficient. In fact, the reaction rates are only in the order of 2 mol/h/l of acid and/or ester produced.
It has been proposed, in order to improve the results of the above-mentioned method, to use the isomerisation reaction in the presence of a strong acid of the sulphonic acid type, such as paratoluenesulphonic acid for example. Under the conditions of this method, the reaction is carried out using significant amounts of methyl formate to be isomerised, which is consequently also used as solvent for the reaction. If this improvement contributes to the improvement of the activity of the reaction, it does nevertheless have the disadvantage of necessitating the use of a further compound, which does not simplify the method.
Further, it is possible that this acid degrades under the conditions of the reaction medium.